Generally, an ignition timing of a spark ignition type internal combustion engine is approximately determined based on an amount of air sucked into a cylinder combustion chamber (intake air amount) and a rotational speed of the internal combustion engine. While, the intake air amount is determined based on a value detected by an airflow meter provided on the upstream side of an intake path.
The intake air amount determined based on the value detected by the airflow meter is sometimes inaccurate and may include an error relative to an actual intake amount sucked into the in-cylinder combustion chamber. That is, the airflow meter is located at a position upstream from the in-cylinder combustion chamber, and if used for an automobile, the internal combustion engine is often in a non-steady state or a transition state rather than in a steady state. Accordingly, there is a time lag between an instant at which air passes the airflow meter and an instant at which it enters the in-cylinder combustion chamber, and in this time interval, it is not rare that the operating state of the internal combustion engine has varied. Therefore, measured value of intake air amount obtained from the airflow meter does not always coincide with the actual air amount sucked in the cylinder combustion chamber. Also, if the internal combustion engine is of a multi-cylinder type, the airflow meter measures the intake air amount at a collecting point before distributing the intake air into the respective cylinders, whereby such a measured value does not always reflect the intake air amount actually sucked into the respective one cylinder. Further, the variation between the respective cylinders caused by the manufacturing error or others may be one factor of the measurement error of the intake air amount.
Since the intake air amount measured by the airflow meter contains the error in such a manner, the ignition timing determined based thereon does not always become an optimum one.
On one hand, a technique for estimating the intake air amount within the in-cylinder combustion chamber not relied on the airflow meter has been proposed in the prior art. For example, in Japanese Patent Application Laid-open No. 2-040054 (1990), a device for controlling an internal combustion engine is disclosed, including means for calculating the variation of the in-cylinder pressure between a timing directly after the opening of an intake valve and a timing directly before the ignition based on the in-cylinder pressure, the crank angle and the opening degree of a throttle and means for calculating the intake air amount from the variation of the in-cylinder pressure and the engine rotational speed.
On the other hand, for improving the output or the efficiency as well as reducing the emission, there has been known an internal combustion engine capable of setting a valve-overlap for simultaneously opening the intake valve and the exhaust valve. In such kinds of internal combustion engines, one provided with a control device has been known, capable of calculating an amount of gas remaining in the combustion chamber (internal EGR amount) caused by the valve-overlap of the intake valve and the exhaust valve (for example, see Japanese Patent Application Laid-open No. 2004-108262). Since the amount of gas actually remaining in the combustion chamber is a sum of an amount of freshly sucked air and an amount of earlier remaining gas, the knowledge of the remaining gas amount is preferable for measuring an accurate amount of intake air (fresh air amount) and determining the optimum ignition timing.
The control device described in Japanese Patent Application Laid-open No. 2004-108262 calculates the in-cylinder temperature and the in-cylinder pressure as well as a gas constant based on signals issued from a discharge temperature sensor, an intake air pressure sensor and a discharge pressure sensor, and calculates an amount of gas in the cylinder at the closing of the exhaust valve based on these in-cylinder temperature, in-cylinder pressure and gas constant. Further, this control device calculates an amount of blowback gas during the overlap of the intake valve and the exhaust valve based on signals issued from a crank angle sensor, a water temperature sensor, a cam angle sensor and an accelerator opening degree sensor, and calculates an amount of gas remaining in the combustion chamber (internal EGR amount) caused by the valve overlap, based on these in-cylinder gas amount and the blowback gas amount.
As described above, according to the conventional control device, it is possible to calculate the amount of intake air in the internal combustion engine and the amount of remaining gas (internal EGR amount) when the valve overlap is set. However, in the above-mentioned prior art, a number of parameters are required for calculating the amount of intake air or the amount of remaining gas. Accordingly, in the conventional internal combustion engine, many sensors are necessary for obtaining these parameters to result in the increase in production cost.